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SubjectRe: WARNING: suspicious RCU usage in ext4_release_system_zone
On Fri, Aug 30, 2019 at 12:28:08PM -0700, syzbot wrote:
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit: ed858b88 Add linux-next specific files for 20190826
> git tree: linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=121b506c600000
> kernel config: https://syzkaller.appspot.com/x/.config?x=ee8373cd9733e305
> dashboard link: https://syzkaller.appspot.com/bug?extid=5bda120b4032f831c57f
> compiler: gcc (GCC) 9.0.0 20181231 (experimental)
>
> Unfortunately, I don't have any reproducer for this crash yet.
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+5bda120b4032f831c57f@syzkaller.appspotmail.com
>
> =============================
> WARNING: suspicious RCU usage
> 5.3.0-rc6-next-20190826 #73 Not tainted
> -----------------------------
> fs/ext4/block_validity.c:333 suspicious rcu_dereference_check() usage!
>

#syz invalid

There was already a fix applied between ed858b88 and latest linux-next:

diff --git a/fs/ext4/block_validity.c b/fs/ext4/block_validity.c
index 003dc1dc2da3..f7bc914a74df 100644
--- a/fs/ext4/block_validity.c
+++ b/fs/ext4/block_validity.c
@@ -330,11 +330,13 @@ void ext4_release_system_zone(struct super_block *sb)
{
struct ext4_system_blocks *system_blks;

+ rcu_read_lock();
system_blks = rcu_dereference(EXT4_SB(sb)->system_blks);
rcu_assign_pointer(EXT4_SB(sb)->system_blks, NULL);

if (system_blks)
call_rcu(&system_blks->rcu, ext4_destroy_system_zone);
+ rcu_read_unlock();
}

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